Radio transmission control system



Mar. 6, 1923. 1,447,773.

' L. ESPENSCHIED ET AL.

RADIOTRANSMISSION CONTROL SYSTEM.

FILED SEPT. 15 1921- W; ATTORNEY Patented er. 6, 1923.

TED STATES LLOYD ESPENSCHIED, OF QUEENS, NEW YORK, AND RALPH BOWN, OF EAST ORANGE, NEW JERSEY, ASSIGNORS TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY,

A CORPORATION OF NEW YORK.

RADIO TRANSMISSION CONTROL SYSTEM.

Application filed September 15, 1921. Serial No. 500,881.

To all whom it may concern:

Be it known that we, LLOYD Esrmvsomno and RALPH BowN, residing at Queens and East Orange, in the counties of Queens and Essex and States of New York and New Jersey, respectively, have invented certain Improvements in Radlo Transmiss on ontrol- Systems, of which the following is a specification.

This invention relates to radio signaling, and more particularly to the control ofthe level of transmission in such s gnaling. While it will be described as applied especially to radio telephony for the reason that the need is greatest in that field, it is to be understood that it may be applied equally well to telegraphy, or the transmission of any other signals. Its ob ect 1s particularly to improve the constancy of transmission characteristics in radlo communlcation.

In radio signaling, there are present to an enormous extent, variations in the transmission characteristics which render it dlflicult to obtain satisfactory signal conditions and which result, among other things, in large fluctuations in the volume of a telephone or other message being transmitted. These variations may be due to such wellknown phenomena as fading away or to differences in day and night signaling. Or again, they may be due to changes 1n the apparatus, either at the sending or receiying stations. If, in long distance rad1o transmission, repeater stat1ons are used, further possibilities for variations enter. These variations may be sudden or slow, depending on their origin, but in any case detract materially from the efi'ectlveness of radio communication.

In our invention we propose to overcome the effect of these variations by providing compensating means, preferably automat c, and in general, to accompllsh th1s we V11- tually set aside a signal channel WhlCh may be considered as a pilot channel over which shall come information as to the transmission character of the ether path through which the signals come. The particular apparatus used to illustrate the invention will be better understood from the following description and the accompanying drawing, in which Figure 1 shows a radio transmission system comprising a transmitting station and a receiving station, the latter showing the invention; Fi 2 is a curve showing the characteristic of the detector which may be used in connection with Fig. 1; Figs. 3, 4 and 5 are modified forms of circuits showing further applications of the invention.

Referring to Fig. 1, at the station T there is located any suitable generator G of high frequency oscillations, which are to be impressed on the radiating antenna 7. These high frequency oscillations are modified in any appropriate manner with the message or signal to be transmitted. At the receiving station R there is shown a receiving antenna 8, including the primary of a transformer 9. A secondary of this transformer, by means of the condenser 10, may be tuned to the carrier frequency. Across the terminals of the condenser 10 there is shown an amplifier 11, which may be of the wellknown three-element vacuum tube type, and the intensity of the high frequency signal impressed on the input of this amplifier may be varied by means of a potentiometer comprising a resistance 12 and swinging arm 13, which makes contact with any one of a number of points of the resistance 12. The output of the amplifier 11, through a transformer 16, is impressed on a detector D, which, for purposes of illustration, has

been shown as a vacuum tube detector 211- though we wish it to be understood that many other forms of detectors are suitable. The low frequency output or signal may then be heard in some indicating device,

such as the telephones T.

One of the properties of a detector of this type as well as of others is that the direct current component varies with the intensity of the impressed high frequency impulses. This is shown, for example, in Fig. 2, in which the space current flowing from plate to filament is plotted against the voltage im-v waves falling on the receiving antenna. If,

i for any reason, the attenuation of the waves passing through the ether is increased, the amplitude of the waves is correspondingly 4 decreased, and the departure of the space current from the normal value ac of Fig. 2 is comparatively small. Use may be made of this fact to increase the gain due to the amplifier 11, and it is our purpose to ad ust this gain. automatically to such an extent that the intensity of the signal received in the indicator I remains substantially constant. We accomplish this by placing in the output circuit of the detector to relays R and R through which the direct current component of the detector flows. The relay R is shown as a lower limit relay and relay R is shown as an upper limit relay. These relays control respectively, the circuits 17 and 18, each containing a relay. Mounted adjacent to these last relays is a switch mechanism consisting of a rocking member 21, which, when tilted in one direction, will make contact with points 22 and 23, and when tilted in the other direction, will make contact with points 2 l and 25. The tiltlng. member 21 is made up of two conductors separated by an insulating block and to these two conductors are connected, respectively, the supply terminals from a suitable source of electric power. The contacts 22 to 25. are connected to a motor 26, which, in turn, by any appropriate mechanism, may

.be used to rotate the arm 13 in the one direction or the other.

The operation of the circuit is as follows: incoming messages are amplified by the amplifier 11 to an extent depending upon the setting of the arm 13 on the potentiometer or gain control. The amplified output impressed on the detector D will produce a signal in the indicator I of a certain volume. The space current through the relays R and R is such that for normal volume neither of the local circuits 17 and 18 is closed. If, however, the attenuation in transmission is such that the signals arriving at the detector D are weak, the spacecurrent will be rela- I tively small and the lower limit relay R will permit the closing of the circuit 17. This circuit, operating upon the rocking member 21, will connect the motor supply through the contacts 22 and 23, causing the motor 26 to turn in such a direction as to step up the arm 13, thereby increasing the gain of' the amplifier .11. This stepping up of the ain will continue until the increase in the irect current component through the relay R is sufiicient to open the circuit 17 In case the intensity of the signals becomes too high, the space current through ,the relay R will be suflicient to close the local circuit 18, which will then tilt the member 21 in a direction to make contact with the points 24 and 25, thereby reversing the connections to the motor-26. This will result in a stepping down of thegainof the amplifier until the volume of the si nal at the indicator I is restored to norma We have thus provided a system which will automatically adjust itself for variations inthe transmission characteristics of the system as a whole in such a manner as to maintain a uniform transmission level, and in this we have virtually made use of the carrier oscillations of the message as a pilot to indicate the state or con dition of the ether through which the waves have been transmitted.

Under extreme conditions of variation it may be that the potentiometer arm 13 may be moved to one extreme limit or the other of the potentiometer 12, and for such cases it is frequently desirable to have notification through some alarm system. For the purposes of illustration we have shown such an alarm system in Fig. 1, consisting of two contact points 28 and 29 in parallel with each other and in series with a lamp 30, and battery 31 which then connects with the arm 13. The adjusted in position that thearm 13 makes contact with the one or the other upon reaching one extreme position or the other. While we have shown the indicator as consisting of a lamp 30, it is obvious that this may be any other suitable alarm device, such as a bell.

We have described the detector D as being one in which there is an increase in the (lirect current component during the reception of high frequency signals. Such detectors may, however, work in the opposite direction, this depending upon the curvature of the characteristic. In the case of a vacuum tube detector, it is usual .to operate it on the lower curve, as shown in Fig. 2. If, however, it should be operated on the upper curve of this characteristic, it is apparent that there would then be a decrease in the direct current component, this decrease being greater as the intensity of the signal increased. In this event, it would obviously be necessary to reverse the connections for the motor 26.

One of the advantages of using the change contacts 28 and 29 would be so in -the direct current component flowing great fluctuations as the message current and therefore apparatus controlled by such change in direct current component is more certain in operation.

A modification of the method used in Fig. 1 is shown in Fig. 3. In this case, instead of using the carrler'frequency as a pilot to indicate the condition of the ether, we propose to modulate the carrier frequency with some suitable lower frequency, thus setting aside a signal channel for ilot purposes. The intensity of this special signal, when received, may be used to control the amplification. In Fig. 3, T again represents the transmitting station w1th the antenna of which is associated a generator G. The high frequency oscillations may be modulated with respect to one or more signal fre quencies or multiplex purposes, as is well understood in the art, and at the same time is modulated with some control frequency. The receiving station R includes an amplifying and gain control mechanism shown conventionally at A, this including, for example, the portion of the circuit included in the dotted lines and indicated A in Fig. 1.

The amplified output is impressed on a detector D in the output circuit of which may be connected appropriate networks S for separating the plurality of signals from each other and from the control frequency. The control frequency may be passed to a rectifier R in series with which is a relay 32, which, through its front and back contacts, is adapted to control the motor 26, this in turn operating the gain control in the one direction or the other as described in connection with Fi 1.

The control frequency may obviously be varied over a wide range but in general it will be desirable to havethis outside the frequency ranges of the signals to be transmitted. If, for example, one of these signals is a telephone signal and the other is a telegraph signal, it would be convenient to give the control frequency some value, such as 5,000. In this modification, the control frequency becomes the pilot channel to indicate the condition of transmission and operates to step up or step down the gain control in accordance with the conditions it may have encountered in arriving at the station B.

Fig. 4 shows a further modification of the invention, in which the high frequency message received at the station R is stepped down by the detector D to some intermediate frequency lower than the carrier frequency and, in general, higher than the speech frequencies, this latter being accomplished by impressing in series on the input of the detector D the received high frequency and locally generated oscillations from a generator 34. The output from the high frequency detector D may then be impressed upon an amplifier A with gain con- I bridged across the output of the amplifier which goes directly to the indicator a circuit containing a rectifier R, the output of which, through a motor control device, is able to control the motor 26 in a manner heretofore described. If the volume of transmission level of the signal passing to the indicator I is below normal, there will be a corresponding decrease in the output of the rectifier R, which will then operate the amplifier A. If, however, the incoming waves are of high amplitude, the increased output of the rectifier R will operate in a compensatory manner on the amplifier A.

.In using the term detector herein, it is; to be taken in a broad sense, as including any device which upon reception of high frequency signals will produce a rectified effect, either electrical or mechanical. It may, for example, be a thermo-junction detector or a hot wire detector in both of which cases a direct current component is produced, dc pendent on the amplitude of the received high frequency waves. On the other hand the detector may,be a hot wire device in which the average change in dimensions depends on the average amplitude of the high frequency waves, and as the mechanical rec-' tified effect varies it may be used to open or close auxiliary circuits to control the amplifier gain in a manner described in connection with various figures. Again, it

may consist of a dynamometer of well known. forms such as that shown in patent to Fessenden, 706,735 of August 12, 1902, the rectified effect being here a mechanical effect which may be used to open and close a circuit. Such a device may obviously be attached to the receiving circuit either ahead of or after the amplifier.

We would not, however, place too much emphasis on the nature o-f-the detecting device but rather on the designation of a radio signal channel to give information as to the transmission character of the signal path. Any means, then, which will indicate the intensity of the incoming waves and control the amplifier gain may be used.

While the invention has been described as operating at the receiver on that receiver, other methods of raising or lowering the transmission level may be used. For example in case of two-way signaling in which there is a transmitting and a receiving equlpment at each station a low volume at a receiver, indicating poor transmission, could be used also to cause the output of more energy from the transmitter at that station, thus raising the transmission level for the return message.

The variations which we have described are for illustrative purposes only. It is to be understood that many other variations may be made in the circuit arrangements, or 1n the details of the motor control and gain control, all of which variations may be made without departing from the spirit or scope of this invention as indicated in the following claims.

What is claimed is:

1. In radio signaling, the method of maintaining. a uniform transmission level, which consists in varying the amplification of the receiver to compensate ations.

2. In a radio signaling system, the method of maintaining a uniform transmission level which consists in using a radio signal channel as a pilot to indicate the transmission character of the transmitting medium and causing this indication to operate on the terminal station apparatus to increase or decrease the power accordingly.

3. In a radio signaling ing a detector, the method of maintaining a uniform transmission level, which consists in causing the change in average current through the detector tion of the signal.

4. In radio signaling, the method of maintaining a uniform transmission level, which consists in amplifying the incoming signal, impressing the amplified signals on a detector, and causing the variations in average detector currents due to transmission changes to vary the amplification accordingly.

5. In a radio receiving system, means for adjusting the transmission level of the received signal, said means being responsive to for transmission varisystem compristo vary the amplifica changes in the" transmission character of the transmitting ether medium.

6.111 a radio receiving circuit comprising a detector, means associated therewith responsive to the intensity of the incoming signal for maintaining a-unform transmission level for the signals.

7. In a radio receivin circuit comprising a detector and an amplifiier, means associated therewith responsive to the intensity of the incoming signal for changing the gain of the amplifier.

8. In a radio receiving circuit comprising a detector and an amplifier, means assohiated therewith responsive to changes in the transmitting character of the transmitting ether medium and adapted to maintain uniform transmission level.

9. In a radio receiving circuit comprising a detector and an amplifier, means associated therewith responsive to the detector for changing the gain of the amplifier.

10. In a radio receiving circuit comprising a detector and an amplifier, means associated therewith responsive to the chan es in the direct current component of the eit lector for changing the gain of the ampli- 11. In a radio receiving circuit comprising an amplifier and a vacuum tube detector, means responsive to the average space current through the detector for changing the amplification of the amplifier.

12. In a radio receiving circuit comprising an amplifier and gain control therefor, and a vacuum tube detector, means responsive to the average space current through the detector forchanging the gain control in accordance with variations in the transmission level.

In testimony whereof, we have signed our names to this specification this 13th day of September, 1921.

LLOYD ESPENSGHIED. RALPH BOWN. 

